if exposed over a 
goodness is felt to 
, but they are ex- 
liable to be moved 
d additional people 
is used, the results 
ition to maintain a 
nts, etc. Due to all 
rs organisation for 
. has been done by 
titudes, eliminating 
nknowns. 
ing contrast trans- 
a “slit” source, and 
give a CT function 
Except for tests on 
st-object remains a 
d, e.g. by construc- 
. This is perhaps a 
sults. The weather 
arope could be used 
y part of the world 
ulness. Nevertheless 
nt the weakest link 
significant advances 
y affect the quality 
s, filters anti-vibra- 
lvancees which have 
; now possible may 
no positive evidence 
the past four years, 
At least two firms 
a towards a general 
some comments are 
orth American mar- 
f the D19 type and 
or variety now com- 
1ave been the stand- 
'ances have made it 
THE PHOTOGRAPHIC IMAGE, BROCK 27 
  
  
  
  
TABLE 3. Characteristics of emulsions available for air photography. 
1 7 
A ; | — > 
Type lvls an FE 
7 | KREBS SS 
Ref. (all panchromatic) Example | (0.2 contrast) | 2 = | 8 SS E x 2 
| lines/mm La [S eim w 5 
| | 
1 Extra high speed Ilford Class N | 24 loz | 15. |éc0m 
| | 
2 |High speed Kodak Super XX | 27 | 13 1.5 660 mu 
Ilford HPA | | 
Eastman Super XX | | 
Eastman Tri X | | 
9 | Medium speed Eastman Plus X | 40 | 0.6 | 2:5 710 mu 
Aerecon | | | 
4 | Low speed Kodak Panatomic X | . 60 0.25 | 1.5 c 
Ilford High | | © = pr 
Resolution | 
  
  
  
  
  
  
possible to offer very marked improvements in resolving power at a moderate sacrifice 
in speed, in the emulsions of reference 4. The better modern wide-angle lenses, with 
greatly improved definition and light-grasp, justify the use of these slower emulsions in 
a variety of photogrammetric tasks. 
The extra high speed types are quite frequently used for tasks where light is poor 
and a large scale minimises the losses due to their lower resolving power. 
Plus X Aerecon represents a new type whose inherently high contrast can be useful 
for small scales and in hazy conditions. 
6.2.2. Speed and resolving power. 
The significant advance during the past few years has been in the slower speed 
range. The drop to one-quarter speed for doubled resolving power shown in Table 1 may 
be compared with the drop to one-tenth which was necessary a decade ago. 
The standard emulsions, reference 2, have substantially the same speed/resolving 
power ratio now as when they were first introduced. (Keeping-quality has been markedly 
improved, however, so that the speed after a year's storage is effectively greater). It is 
a matter for discussion whether the emulsion makers should be encouraged to strive, in 
this class, for greater speed with the same resolving power, or the converse. The writer 
would favour the second alternative, but would insist on retention of the good-keeping 
qualities whatever might be done. 
So far there appears to be no need for emulsions of higher resolving power than 
those of reference 4, because they do not limit the off axis performance of even the best 
lenses. 
6.2.8. Gamma. 
A gamma of about 1.5 suits many situations in air photography, and this is given 
by the standard reference 2 emulsions at full development in a D19 type solution, which 
also gives maximum speed. Strictly speaking we should use some figure expressive of 
average gradient, rather than gamma, because characteristic curve shapes are not neces- 
sarily the same for all emulsions. In practice however, they vary little and gamma is 
convenient and not too inaccurate. 
Choice of gamma is of course a very arbitrary matter, depending on the luminance 
range of the scene, the technique of printing, the photographer's skill in estimating ex-